Using atomistic simulations, we investigate the dynamical behavior of a single skyrmion interacting with an asymmetric linear protrusion array under external ac driving. The linear protrusion array is placed in the $xy$ plane and is composed of magnetic walls near the edge of the nanotrack that have angled linear protrusions into the nanotrack. The structure forms a hard axis for $-x$ direction motion and an easy axis for $+x$ direction motion. When the ac drive is applied along the $x$ direction, the skyrmion moves along the hard direction of the substrate asymmetry in three phases: a pinned phase with localized skyrmion orbits, a constant velocity phase where the orbits become delocalized, and a reentrant pinned phase with larger localized orbits. We measure the dependence of the skyrmion velocity on the frequency and amplitude of the ac drive. All three phases appear for all frequency values simulated here, and in the constant velocity phase the skyrmion velocity depends only on the frequency and not on the amplitude of the ac drive. When ac driving is applied in the $y$ direction, the skyrmion moves along the easy direction of the substrate asymmetry and exhibits the same three phases as for $x$-direction driving along with a fourth phase which, at high driving frequencies, consists of a series of constant velocity phases, each with different average skyrmion velocities. For low frequencies, the constant velocity phase is lost and the skyrmion speed increases linearly with increasing ac drive amplitude due to a Magnus boost effect. Our findings suggest new ways to create reliable data transport for spintronic devices using skyrmions as information carriers, where the skyrmion direction and speed can be controlled by varying only the ac drive amplitude and frequency.

A general theory for the melting of two-dimensional (2D) solids that explains the universal and nonuniversal properties remains an open problem. Although the celebrated Kosterlitz-Thouless-Halperin-Nelson-Young theory was able to predict the critical properties of the melting transition in a variety of cases, it is already known that it could not capture the occurrence of first-order transitions observed in certain systems, nor does it provide a clear way to calculate the melting temperature for a specific model. In the present work, we have developed an analytical method that combines the self-consistent variational approximation with the renormalization group in order to deal simultaneously with the phonon fluctuations and the topological defects that are present in the melting process of 2D crystals. The method was applied with impressive success to a study of the phase diagram of the Gaussian core model, capturing not only the reentrant feature of its 2D solid phase, but also the related critical temperatures as a function of the density in quantitative detail. The developed method can be directly applied to study the melting of any hexagonal simple crystal formed by particles interacting through any finite pairwise interaction potential. Additionally, it has the potential to explain the occurrence of first-order transitions in the melting process of 2D crystals.

We consider a family of quantum many-body Hamiltonians that show exact Hilbert space fragmentation in certain limits. The question arises whether fragmentation has implications for Hamiltonians in the vicinity of the subset defined by these exactly fragmented models, in particular in the thermodynamic limit. We attempt to illuminate this issue by considering distinguishable classes of transitional behavior between fragmented and nonfragmented regimes and employing a set of numerical observables that indicate this transition. As one of these observables we present a modified inverse participation ratio (IPR) that is designed to capture the emergence of fragmented block structures. We compare this block IPR to other definitions of inverse participation ratios, as well as to the more traditional measures of level-spacing statistics and entanglement entropy. In order to resolve subtleties that arise in the numerics, we use perturbation theory around the fragmented limit as a basis for defining an effective block structure. We find that our block IPR predicts a boundary between fragmented and nonfragmented regimes that is compatible with results based on level statistics and bipartite entanglement. A scaling analysis indicates that a finite region around the exactly fragmented limit is dominated by effects of approximate fragmentation, even in the thermodynamic limit, and suggests that fragmentation constitutes a phase. We provide evidence for the universality of our approach by applying it to a different family of Hamiltonians, that features a fragmented limit due to emergent dipole conservation.

Finding and understanding non-Fermi-liquid transport behaviors are at the core of condensed matter physics. Most of the existing studies in this field were devoted to the monolayer Hubbard model, which is the minimal model that captures the essential features of high-temperature superconductivity. Here, we discover another type of non-Fermi-liquid behavior emergent in the hole-doped bilayer Hubbard model, using dynamical mean-field theory with a full consideration of the short-range interlayer electron correlation. We find that at low temperatures, the Hall coefficient has a strong nonmonotonic dependence on temperature, leading to a double or quadruple reversal of its sign depending on the doping level. At the same time, the resistivity exhibits two plateaus rather than linearity in its temperature dependence. We show that these intriguing transport behaviors stem from the formation of coherent interlayer singlets, which scatter off gapped collective modes arising from short-range interlayer antiferromagnetic fluctuations.

In recent years, two-dimensional (2D) second-order topological insulators (SOTIs) have garnered significant interest, with indications of their potential realization in various symmorphic 2D electronic materials. However, up to this point, no nonsymmorphic 2D electronic SOTIs have been identified, probably due to the inability of nonsymmorphic operations to maintain the invariance of nanoflakes. In this paper, we investigate the existence of nonsymmorphic 2D SOTIs, unveiling hidden higher-order topology within 2D nonsymmorphic electronic systems. Our findings are substantiated by symmetry analyses, tight-binding (TB) models, and first-principles calculations. The emergence of topological corner states in these nonsymmorphic 2D SOTIs is attributed to the filling anomaly within a set of symmorphic Wannier orbitals, which exhibit a symmorphic distribution. We identify square-octagon monolayers (so-MLs) of group IV and V elements, including 2D tetragonal P, and 2D hydrogenated tetragonal Si and Ge, as promising material candidates. The corner states in these nonsymmorphic so-MLs are protected by a point symmetry ($C_4$ rotation). The TB model of so-MLs behaves similarly to the Su-Schrieffer-Heeger model, with higher-order topological insulating phases having greater intersquare hoppings compared to intrasquare hoppings, while the reverse is considered trivial. These discoveries not only enrich our theoretical comprehension of higher-order topology but also introduce potential material candidates for experimental exploration, thus advancing the field of topological crystalline materials.

We analyze a mesoscopic conductor autonomously performing a thermodynamically useful task, such as cooling or producing electrical power, in a part of the system—the working substance—by exploiting another terminal or set of terminals—the resource—that contains a stationary nonthermal (nonequilibrium) distribution. Thanks to the nonthermal properties of the resource, on average no exchange of particles or energy with the working substance is required to fulfill the task. This resembles the action of a demon, as long as only average quantities are considered. Here, we go beyond a description based on average currents and investigate the role of fluctuations in such a system. We show that a minimum level of entropy fluctuations in the system is necessary, whenever one is exploiting a certain entropy production in the resource terminal to perform a useful task in the working substance. For concrete implementations of the demonic nonthermal engine in three- and four-terminal electronic conductors in the quantum Hall regime, we compare the resource fluctuations to the entropy production in the resource and to the useful engine output (produced power or cooling power).

The development of active metasurface systems, such as lasing metasurfaces, requires the optimization of multiple modes at the absorption and lasing wavelength bands, including their quality factor, mode profile, and angular dispersion. Often, these requirements are contradictory and impossible to obtain with conventional design techniques. Importantly, the properties of the eigenmodes of a metasurface are directly linked to their symmetry, which offers an opportunity to explore mode symmetry as an objective in optimization routines for active metasurface design. Here we propose and numerically demonstrate a multiobjective optimization technique based on symmetry projection operators to quantify the symmetry of the metasurface eigenmodes. We present, as an example, the optimization of a lasing metasurface based on up-converting nanoparticles. Our technique allows us to optimize the absorption mode dispersion, as well as the directionality of the lasing emission, and therefore offers advantages for novel lasing systems with high directionality and low lasing threshold.

Depth-dependent electronic and magnetic states of $\mathrm{Al}{\mathrm{O}}_x$ and MgO capped ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ thin films were measured by using hard x-ray photoemission spectroscopy (HAXPES) combined with x-ray total reflection (TR). TR-HAXPES revealed that the near-interface electronic and magnetic states of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ films differed from those of bulk measured in non-TR condition. The decrease of the Co and Mn magnetizations near the interface along the easy magnetization axis in the bulk region relative to those in the bulk region and the changes in the valence band profiles were experimentally detected by nondestructive HAXPES utilizing TR. The possible origin of the reduction of the Co and Mn magnetizations and the changes in the valence band profile near the $\mathrm{Al}{\mathrm{O}}_x/{\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ interface was due to an enhanced spin-wave excitation originating from the weakened exchange interaction between the local magnetic moments compared to the bulk region of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$, which can slightly modify the valence band electronic states, near the interface at a finite temperature. These results suggest that the combination of HAXPES with TR is useful to experimentally detect the electronic and magnetic states of near-interface and buried bulk regions in nondestructive way for insulator/ferromagnet heterojunctions.

The Co-intercalated ${\mathrm{NbS}}_2$ (${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$) compound exhibits large anomalous Hall conductance, likely due to the noncoplanar magnetic ordering of Co spins. In this work, we study the relation between this novel magnetism and the correlated electronic structure of ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ by adopting dynamical mean-field theory (DMFT) to treat the correlation effect of Co $d$ orbitals. We find that the hole doping of ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ can tune the size of the Nb hole pocket at the DMFT Fermi surface, producing features consistent with those observed in angle-resolved photoemission spectra [H. Tanaka et al., Phys. Rev. B 105, L121102 (2022)]. We also compute the momentum-resolved spin susceptibility and correlate it with the Fermi surface shape. We find that the magnetic ordering wave vector of ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ obtained from the peak in spin susceptibility agrees with the one observed experimentally by neutron scattering; it is compatible with a commensurate noncoplanar $3q$ spin structure. We also discuss how results change if other than Co transition-metal intercalations are used.

Hybrid systems comprising superconducting and semiconducting materials are promising architectures for quantum computing. Superconductors induce long-range interactions between the spin degrees of freedom of semiconducting quantum dots. These interactions are widely anisotropic when the semiconductor material has strong spin-orbit interactions. We show that this anisotropy is tunable and enables fast and high-fidelity two-qubit gates between singlet-triplet (ST) spin qubits. Our design is immune to leakage of the quantum information into noncomputational states and removes always-on interactions between the qubits, thus resolving key open challenges for these architectures. Our ST qubits do not require additional technologically demanding components nor fine-tuning of parameters. They operate at low magnetic fields of a few millitesla and are fully compatible with superconductors. By suppressing systematic errors in realistic devices, we estimate infidelities below ${10}^{-3}$, which could pave the way toward large-scale hybrid superconducting-semiconducting quantum processors.

Higher-order topological phases of matter, including insulators and semimetals, have attracted much attention, since they can induce novel multidimensional topological boundary states. Recently, a two-dimensional (2D) multipole chiral-symmetric higher-order topological insulator (MCTI) with multipole corner states, protected by multiple chiral numbers (MCNs), has been proposed theoretically and soon will be implemented in circuit and acoustic systems. However, the chiral-symmetry higher-order topological semimetals remain unexplored. In this work, we study the multipole chiral symmetry on the three-dimensional topological semimetals based on long-range hoppings. We theoretically propose two types of multiple chiral topological semimetals (MCTSs). The multipole hinge states, which are protected by $k_z$-dependent MCNs, are observed at one-dimensional (1D) hinge boundaries in the corresponding subspace. To verify our theory, we construct one type of MCTS in acoustic crystal as an example. Four groups of topological hinge states are obtained at each 1D hinge boundary of the constructed acoustic system. These multiple hinge states may have potential applications to improve the sensing and calculation of quantum devices.

We study non-Hermitian (NH) fermionic superfluidity subject to dissipation of Cooper pairs on a honeycomb lattice, for which we analyze the attractive Hubbard model with a complex-valued interaction. Remarkably, we demonstrate the emergence of the dissipation-induced superfluid phase that is anomalously enlarged by a cusp on the phase boundary. We find that this unconventional phase transition originates from the interplay between exceptional lines and Van Hove singularity, which has no counterpart in equilibrium. Moreover, we demonstrate that the infinitesimal dissipation induces the nontrivial superfluid solution at the critical point. Our results can be tested in ultracold atoms with photoassociation techniques by postselecting special measurement outcomes with the use of quantum-gas microscopy, and they can lead to an understanding of the NH many-body physics triggered by exceptional manifolds in open quantum systems.

We present an atomistic approach, based on the x-ray diffraction structure, to model cooperative spin-transition phenomena in the $\left[\mathrm{Fe}\right(\text{pyrazine}\left)\right] \left[\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right]$ spin-crossover compound. The vibronic coupling is described through a double-well potential along the totally symmetric Fe-ligand stretching mode, whereas the elastic interactions between the Fe centers are considered by an additional spin-state-dependent two-body potential. The model is then investigated through molecular dynamics simulations in the isothermal-isobaric ensemble. This approach provides a real-time spatiotemporal description of the spin transition, from the atomic movements to the nanoscale behavior, removing in this way some ad hoc assumptions used in state-of-the-art atomistic models, while keeping the computational cost affordable. This work is separated into two papers. In the present Part I, we report on the methodology used to describe the electron-lattice interaction to simulate the spin transition in the bulk material within a realistic molecular structure. Part II [S. Mi et al., Phys. Rev. B 109, 054104 (2024)] will address the spatiotemporal dynamics (nucleation and growth) of the spin transition in a bilayer actuator, correlating the buildup of elastic stresses and the resulting deformation of the nanoscale object using the atomistic approach developed here.

We present an atomistic approach, based on a realistic structure of the compound $\left[\mathrm{Fe}\right(\text{pyrazine}\left)\right][\mathrm{Ni}{\left(\mathrm{CN}\right)}_4]$, which provides a spatiotemporal description of the spin-crossover (SCO) phenomenon. The vibronic coupling is described through an intramolecular double-well potential, whereas the elastic interactions between the Fe centers are considered by an additional spin-state-dependent potential. The model is then investigated through molecular dynamics (MD) simulations. This work is separated into two papers. Part I [S. Mi et al., Phys. Rev. B 109, 054103 (2024)] reports on the methodology used to simulate the spin transition in the bulk material. The present Part II addresses the spatiotemporal dynamics (nucleation and growth) of the spin transition in a bimorph actuator, mimicking the operation of a nanoelectromechanical device. The spin-state configuration and local strain are calculated to explore both the spatiotemporal aspects of the spin-state switching and the resulting deformation of the bimorph cantilever, as a function of the cooperativity of the SCO material and the thickness of the films. We show that the spatiotemporal dynamics deviates from a single nucleation process since additional domains can be formed due to the lattice bending, whereas the associated elastic stresses lead to longer switching times and to incomplete transitions. A comparison with classical continuum mechanical theory reveals that the latter may be no more valid in the case of systems with ultrathin films. Interestingly, due to competing effects between the spin-state switching dynamics and the natural frequencies of the cantilever, damped oscillations are observed in the spin-state fraction and in the deformation of the cantilever before reaching a stationary state.

We show that the internal quality factors of high-impedance superconducting resonators made of granular aluminum can be improved by coating them with micrometric films of solid parahydrogen molecular crystals. We attribute the average measured $\approx 8\%$ reduction in dissipation to the absorption of stray terahertz radiation at the crystal-resonator interface and the subsequent dissipation of its energy in the form of phonons below the pair-breaking gap. Our results prove that contrary to expectations, replacing the vacuum dielectric atop a superconducting resonator can be beneficial, thanks to the added protection against Cooper pair-braking terahertz radiation. Moreover, at the level of internal quality factors in the ${10}^5$ range, the hydrogen crystal does not introduce additional losses, which is promising for embedding impurities to couple to superconducting thin-film devices in hybrid quantum architectures.

We investigate the single-particle spectra of recently discovered kagome superconductors. We examine nine distinct superconducting pairing symmetries, including the uniform $s$-wave pairing function, nearest-neighbor and next-nearest-neighbor $s$-wave pairing, $p+ip$ pairing, $d+id$ pairing, and $f$-wave pairing states. These pairing states can be classified into irreducible representations of the $C_{6v}$ point group. Our findings suggest that these pairing states can be differentiated by analyzing the energy bands, spectral function, and local density of states. Additionally, we explore the single impurity effect, which could potentially assist in further distinguishing these pairing symmetries.

Néel magnetic domain walls that are stabilized by achiral energy terms instead of the usual Dzyaloshinskii-Moriya interaction will be bistable, with the two possible chiral forms being degenerate. Here we focus on the theoretical study of the spin-orbit torque driven dynamics of such bistable Néel domain walls. We find that, for a given domain wall, two propagation directions along a nanowire are possible, depending on its initial state. These dynamics also exhibit complex dependence on the spin-orbit torque magnitude, leading to important transient regimes. Finally, a few ways are proposed for controlled or random reversal of the domain wall propagation direction. A robust analytical model which handles all the observed behaviors of such domain walls is developed and validated by comparing with numerical simulations. The dynamics obtained by this approach opens the way for using domain walls in information storage and processing devices in ways that are not possible with fixed chirality walls.

Inspired by recent advancements in the bulk photovoltaic effect which can extend beyond the independent-particle approximation (IPA), this study delves into the influence of collective spin dynamics in an antiferromagnet on photocurrent generation using a time domain calculation. In the linear and photocurrent conductivity spectra, we observe peaks below the band gap regime, attributed to the resonant contributions of collective modes, alongside broadband modifications resulting from off-resonant spin dynamics. Notably, the emergence of spin dynamics allows various types of photocurrent, which are absent in the IPA framework. Furthermore, we emphasize the importance of energy scale proximity between electronic and spin degrees of freedom in enabling efficient feedback. These findings offer new avenues for efficient energy harvesting and optoelectronic applications.

We study the behavior of linear and nonlinear spectroscopic quantities in two-dimensional topologically ordered systems, which host anyonic excitations exhibiting fractional statistics. We highlight the role that braiding phases between anyons have on the dynamics of such quasiparticles, which as we show dictates the behavior of both linear response coefficients at finite temperatures, as well as nonlinear pump-probe response coefficients. These quantities, which act as probes of temporal correlations in the system, are shown to obey distinctive universal forms at sufficiently long timescales. As well as providing an experimentally measurable fingerprint of anyonic statistics, the universal behavior that we find also demonstrates anomalously fast thermal relaxation: correlation functions decay as a “squished exponential” $C\left(t\right)\sim exp(-{[t/\tau ]}^{3/2})$ at long times. We attribute this unusual asymptotic form to the nonlocal nature of interactions between anyons, which allows relaxation to occur much faster than in systems with quasiparticles interacting via local, nonstatistical interactions. While our results apply to any Abelian or non-Abelian topological phase in two-dimensions, we discuss in particular the implications for candidate quantum spin liquid materials, wherein the relevant quantities can be measured using pre-existing time-resolved terahertz-domain spectroscopic techniques.